Multispecies Methods of Testing for Toxicity: Use of the Rhizobium-Legume Symbiosis in Nitrogen Fixation and Correlations Between Responses by Algae and Terrestrial Plants

Responses of the Rhizobium-legume symbiotic relationship to long-term (5- to 7-week) and short-term (2-week) exposures of copper sulfate, 2,4-dichlorophenoxyacetic acid, and streptomycin sulfate were examined in bush beans and clover. Toxic effects were evaluated by comparing plant biomass (yield), nodulation success, nitrogen fixation rate as indicated by acetylene reduction, and plant nitrogen content in controls and in plants exposed to various concentrations of the chemicals. Plants with long-term continuous root exposure were affected more than plants with short-term exposure, as indicated by reductions in nitrogen fixation rates and plant growth. Although rates of acetylene reduction (nitrogen fixation) were depressed, plant biomass and the numbers of root nodules produced were simpler, less expensive indices of exposure.

In a second test, the responses of algae (Selenastrum capricornutum and Chlorella vulgaris) and terrestrial plants (radishes, barley, bush beans, and soybeans) to 21 different herbicides were compared to evaluate the use of a short-term (96-h) algal growth inhibition test for identifying chemicals potentially toxic to terrestrial plants. Two test end points were evaluated: (1) the highest concentration resulting in no statistically significant reduction in the growth rate of algae or terrestrial plants, and (2) the concentration at which algal growth or terrestrial plant biomass was reduced by 50%. Test results were correlated for the two algae and for all pairs of vascular plants. However, results of the algal tests did not correlate with those of the terrestrial plants. Furthermore, according to tests with Selenastrum capricornutum, there was only a 50% chance of successfully identifying herbicide levels that reduced terrestrial plant biomass. For the herbicides and species examined, short-term toxicity tests with algae were not good indicators of toxicity to terrestrial plants.